1. Field of the Invention
The present invention relates to a method of detecting a deteriorated condition of a wide range air-fuel ratio sensor, i.e., whether a wide range air-fuel ratio has been deteriorated or not. The present invention further relates to an apparatus for carrying out such a method.
2. Description of the Related Art
For controlling an air-fuel ratio mixture to be supplied to an engine in a way as to allow the air-fuel ratio to be maintained at a target value (i.e., stoichiometric) and thereby reducing the concentration of CO, NOx, and HC in the engine exhaust gases, it is known to carry out a feedback control of a quality of fuel to be supplied to the engine. Mainly used for such feedback control is a .lambda. (lambda) sensor whose output changes abruptly or sharply (i.e., stepwise) in response to a particular oxygen concentration, i.e., a theoretical air-fuel ratio mixture, and further is a wide range air-fuel ratio sensor or oxygen sensor, whose output changes smoothly and continuously (i.e., not stepwise) in response to a variation of the air-fuel ratio from a lean mixture mode or range to a rich mixture mode or range. The wide range air-fuel ratio sensor, as mentioned above, is capable of detecting the oxygen concentration in an engine exhaust gas continuously and improving the feedback control accuracy and speed, and is thus used in case the higher-speed and more accurate feedback control is required.
The wide range air-fuel ratio sensor is provided with two cells which are made of oxygen ion conductive solid electrolytic bodies and disposed so as to oppose each other with a certain interval or gap (measurement chamber) therebetween. One of the cells is used as a pump cell for pumping out the oxygen from or into the gap between the cells. The other of the cells is used as an electromotive force cell for generating a voltage depending upon a difference in the oxygen concentration between an oxygen reference chamber and the above gap. The pump cell is operated in such a manner that the output of the electromotive force cell is constant, and the current supplied to the pump cell to this end is measured for use as a value proportional to a measured oxygen concentration. An example of such a wide range air-fuel ratio sensor is disclosed in U.S. Pat. Nos. 5,174,885 and 5,194,135.
The above described feedback control for reducing the noxious components contained in the exhaust gases starts after the engine has warmed up. This is because the wide range air-fuel ratio sensor is not active or operable until it is heated up to a predetermined temperature to make higher the activity of its oxygen ion conductive solid electrolyte. For this reason, a heater is provided to the wide range air-fuel ratio sensor in order to make it operable as soon as possible after starting of the engine.
In this connection, before starting of the feedback control by the above described wide range air-fuel ratio sensor, the air-fuel ratio is, in many cases, regulated to a rich mode with a view to preventing stopping of the engine such that the exhaust gases with a relatively high concentration of CO and HC are emitted. In order that the wide range air-fuel ratio sensor can be put into action as early as possible after starting of the engine so that the emission of such exhaust gases with a high concentration of noxious components is terminated within a short time, judgment on whether the wide range air-fuel ratio sensor has been activated or not is made by applying a predetermined current to the electromotive force cell for measurement of the resistance.
The electromotive force cell has a negative temperature-resistance characteristic, so its resistance becomes gradually smaller as it is heated up to a higher temperature by a heater. Namely, from the fact that the electromotive force cell has been reached a temperature at which it becomes active or operable, it is judged that the wide range air-fuel ratio sensor is in condition of being capable of starting measurement.
In this connection, deterioration is not caused in the oxygen ion conductive electrolytic body constituting the electromotive force cell of the wide range air-fuel ratio sensor but in the porous electrode made of Pt (platinum) or the like and attached to the electromotive force cell and in the interface between the solid electrolytic body and the porous electrode. Namely, the porous electrode is separated from the oxygen ion conductive solid electrolytic body or reduces in the oxygen permeability after a certain period of usage of the sensor, thus increasing in the internal resistance and deteriorating gradually.
When the deterioration has advanced above a certain degree, here arises a problem that it becomes impossible to carry out accurate detection of the air-fuel ratio. Up to now, there has not been known a method that can detect deterioration of such a wide range air-fuel ratio sensor accurately.